1. Field of the Invention
The present invention generally relates to the field of disk drives, and in particular, to an error correction technique at the time of a read operation.
2. Description of the Related Art
Generally, disk drives, a typical example of which is a hard disk drive, record data on a disk medium and read data from the disk medium by means of a head. In this read operation for reading data, the data is reproduced (decoded) by a read channel from a data signal output from the head into user data which is the original recorded data.
In the read operation, there are cases in which an error arises in a data bit row included in the data signal due to effects such as noise. In a normal disk drive, ECC data which is necessary for error detection and correction (ECC) processing is added to user data (data in units of sectors) recorded on a disk medium. A disk controller (HDC) of the drive has an error correction unit which executes the error detection/correction processing by using the user data and the ECC data output from the read channel.
The error correction unit of the HDC can exhibit a good error correction ability if error positional information showing the error position at which an error occurred in the user data can be acquired. Among read channels in recent years, there has been developed a read channel including an erasure pointer (hereinafter called “EP unit”) generating error positional information from the decoded output of the user data.
The EP unit generates error positional information in which, for example, a bit position which cannot be distinguished as either “0” or “1” in a bit row of the data is made to be erasure. Namely, the error positional information is information showing an error position at which there is a high possibility that an error has occurred. However, there are cases in which information, in which an area where an error has actually not occurred is also indicated as an error position, is included in the error positional information generated by the EP unit. In the error correction unit, if error positional information having low reliability is used, there is the possibility that the error correction ability will deteriorate more than in the case in which error positional information is not used.